Most existing telecommunication systems operate on a limited audio bandwidth stemming from limitations of land-line telephony systems. Typically, for most voice services only the lower end, i.e., the low-frequency part, of the audio spectrum is transmitted.
Although the limited audio bandwidth is sufficient for most conversations, there is a desire to increase the audio bandwidth to improve intelligibility and sense of presence. Despite the fact that the capacity in telecommunication networks is continuously increasing, it is still of great interest to limit the required bandwidth per communication channel. In particular in mobile networks, smaller transmission bandwidths for each call result in a reduced power consumption in both the mobile device and the base station. This translates to energy and cost saving for the mobile operator, while the end user will experience prolonged battery life and increased talk-time. Further, with less consumed bandwidth per user, the mobile network can serve a larger number of users in parallel.
A property of the human auditory system is that the perception of sound is frequency dependent. In particular, our hearing is less accurate at higher frequencies. This has inspired so-called bandwidth extension (BWE) techniques, which are based on reconstructing a high-frequency band from a low-frequency band, and possibly also on a low number of high-band parameters, transmitted from the encoder side to the decoder side.
Since BWE is typically performed with limited resources, the perceived quality of the extended frequency region may vary. In 0-bit BWE schemes, i.e. in which no high-band parameters are transmitted from the encoder to the decoder side, it is common to attenuate the global gain of the BWE signal by scaling with a constant, i.e. multiplying all samples of the BWE signal by a constant attenuation factor, in order to conceal artifacts caused by the BWE system. However, the attenuation of the global gain of the BWE signal will also reduce the sensation of presence of the signal.